Various types of control systems, especially control systems for use with automotive vehicles have been proposed. Typical automotive-type electronic control systems use microprocessors which include a central processing unit, read-only-memories (ROMs) and buffer memories, such as random access memories (RAMs). It has also been proposed to include in the system an auxiliary ROM, which permits correction of data or change of data. Change of data or correction of data is desirable under some circumstances, for example, if a specific engine is to be associated with a vehicle of a different type, for example of a different weight, or if the model or series of the vehicle and/or of the engine changes. The presence or absence of such an auxiliary ROM can be determined by storing, in a predetermined address, a characteristic data word, for example a single logic-1 already during programming of the system; the address of the auxiliary data word is then interrogated by the program cycle which is run by the central processing unit (CPU) of the microprocessor. Such an auxiliary ROM is used to change data within the main ROM, in order to, for example, match specific operating data stored in the ROM to a specific engine. It is possible, for example, to recognize different constants or different vectors or different functional relationships or tables. It is not possible, however, to change the overall program cycle since the sequence of programming steps carried out by the CPU is based on the memory content within the main ROM.